1. Field of the Invention
The present invention relates to an optical information processing system and, more particularly, to an improvement in an optical system of an optical head assembled in an optical information processing system.
2. Description of the Related Art
In an optical information processing system, e.g., an optical filing apparatus, an optical head is used to record information in an optical information recording medium, i.e., an optical disk, and to reproduce the information therefrom. In such an optical head, a light beam emitted from a laser diode serving as a light source is focused on the optical disk by an objective lens in the optical head, and the light beam reflected by the optical disk is guided to the detector. This beam is detected, and is converted into a retrieve signal. In a reproducing/recording operation of information from/in such an optical disk, the objective lens is maintained in an in-focus state, and a beam waist, i.e., a minimum beam spot, of the light beam is formed on the optical disk. In addition, the objective lens is maintained in an in-track state, and a tracking guide formed on the optical disk is traced by the light beam. Information is accurately recorded in or reproduced from the optical disk. Among methods of detecting such an in-focus or in-track state, a knife-edge method, an astigmatism method, and a push-pull method are conventionally known. In the knife-edge method, a light beam returned from the optical disk is partially extracted by a knife edge arranged on an optical path, and the extracted beam is projected on a focusing detection photodetector through a focusing detection optical system. In this knife-edge method, since a light beam spot formed on a photodetector to detect a focal point is displaced in accordance with a focusing state of the objective lens, the focusing state of the objective lens can be detected. In the astigmatism method, an astigmatism is applied to the light beam returned from the optical disk by the astigmatism lens of the focusing detection optical system arranged on the optical path, and the obtained light beam is projected on the focusing detection photodetector. In this astigmatism method, the shape of a light beam spot formed on the focusing detection photodetector is changed in accordance with a focusing state of the objective lens to detect the focusing state of the objective lens. In the push-pull method, a light beam is diffracted on and returned to the optical disk by the tracking guide, and the returned light beam is projected on a tracking guide detection photodetector. In this push-pull method, an image or a diffraction pattern of a tracking guide is formed in the beam spot detected on the tracking guide detection photodetector as a dark portion. Therefore, when the position of the dark portion is detected, a tracking state of the objective lens can be detected.
In the optical system of the optical head, the larger the focal length of the focusing detection optical system is, the more the detection sensitivity is improved. On the contrary, the larger the focal length of the focusing detection optical system is, the larger the size of the optical head. When the size of the optical head is increased, not only the size of the apparatus itself but also a load of a drive mechanism for the optical head are increased, and it is difficult to access information at high speed. In addition, if a convex lens having a large curvature is assembled in the focusing detection optical system in order to achieve a compact apparatus, the convex lens applies a spherical aberration to the focusing detection light beam, and it is difficult to achieve accurate focusing detection.
The present inventors have already proposed an optical system associated with a so-called twin plate method which is different from the above-mentioned focusing detection system in U.S. Ser. No. 271,827, Ishika filed Nov. 16, 1988 (corresponding EPC application No. 88119021.9, filed Nov. 15, 1988 ). In this optical system, similarly, the larger the focal length of the focusing detection system optical system is, the further the detection sensitivity is improved. On the contrary, the larger the focal length of the focusing detection optical system is, the larger the size of the optical head, and a demand has arisen for improving the conventional system.